Static mixers are commercially available and are used for mixing liquid/liquid or gas/liquid. The geometrical design of the mixers may vary but the main principle for the mixing is mainly the same. The mixer comprises stationary (unmovable) mixing elements and the media which are to be mixed are pumped to and through the mixer. From the beginning static mixers were from the beginning developed mainly for laminar flow. With laminar a flow mixing takes place when the mixing elements divide and divert the streams of different media a number of times. The necessary energy for the mixing is taken from the pumping energy. The number of mixing elements that are needed other things to obtain a desired degree of mixing depends among all of the mixing ability of the media. A higher number of elements are needed for more difficult mixing duty.
Early examples of static mixers are those described, for example, in U.S. Pat. No. 3,051,452 and U.S. Pat. No. 3,206,170. In the first mentioned patent there is described a static mixer which divides a flow through a tube into two concentric flows which are forced to flow radially inwardly and outwardly while the flows are divided into different partial streams. U.S. Pat. No. 3,206,170 describes mixing of two flows which are brought to flow through a number of elements with rectangular channels which are widened and divided into new channels which are widened and so on.
A static mixer with a low pressure drop is described in U.S. Pat. No. 3,286,992. This mixer comprises twisted mixing elements arranged within a tube. The mixing elements consist of discs which are twisted 60.degree.-210.degree.. The discs extend to the walls of the tube and divide the same into two separate channels. Each twisted disc is connected to the next disc. Static mixers with similar twisted elements are described in for example GB 1 386 955 and GB 2 127 316.
The static mixers described above work well for clean liquids but for liquid products comprising particles problems may arise as a consequence of the fact that the twisted mixing elements have sharp edges which may create blocking and problems with cleaning. This applies to an even greater extent if the material consists of food products containing fruit or vegetables which may be damaged by too sharp edges.